I am writing a web application prototype for computer assembly and was wondering which components are essential. I do not have the time to include every component, as I would need to get database data for them as well.

So far I have:
MoBo, CPU, Video Card, RAM, HD, Optical Drive, Monitor

Do I have to include a PSU, Case, Keyboard, Mouse?

Also, if anyone knows of a database of computer parts that I can use, that would be greatly appreciated. It does not have to be extensive or up-to-date.

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1

Video card, hard disk, optical drive and monitor are, in fact, optional. It depends on what you want to do with the computer.
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Daniel Beck♦Nov 12 '11 at 13:16

nothing, i'm writing a assembly online store app that will never go online. the question is what could i leave out without sacrificing too much? cpu+mobo+ram is a bit... not enough
–
glassNov 16 '11 at 17:15

In that case, do whatever other stores are doing, or just what feels right to you. It also sounds way too localized a problem (as it's not an actual computer assembly problem you're facing).
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Daniel Beck♦Nov 16 '11 at 17:24

Those are the most important parts on your motherboard, nicely put into an ASCII drawing for you :p
CPU connected to the north bridge via the Front Side Bus (FSB). The north bridge is connected to the AGP slot and the RAM memory, and to the south bridge, which is connected to the USB hubs, SATA/IDE cables, and PCI slots. The hard drives and CD/DVD drives are connected with the IDE/SATA cables. In some computers there's also a Bergh connector for a floppy drive.
In short terms, those are the most important internal components of the computer. The case is also essential for protecting the compontents against static electricity, and of course the power supply, or the computer wouldn't be able to do anything.
And of course, to be able to use a computer fluently, you'll need a monitor, mouse, and keyboard.

The bare minimum you could run a system off is a stick of ram,CPU, motherboard (with some way to switch it on, i've used a 2p coin) power supply, and some boot media (network, or USB would do). Of course, this would be a headless system, with no input, and would suck to do anything at all without pre-configuration. Running a system without a case also brings in other issues, such as possible EMI, and the risk of getting something caught in a fan.

On the other hand, for the purposes of the actual capabilities of a system, pretty much anything other than ram, cpu and hard drive is not hugely important.I'd probably suggest adding power supply rating and OS if its a environment with many OSes.

For the second part of your question see this question - in short, you could scrape it off retailers or wikipedia.How specific you are would also be an issue - are we going by component type (which would be simple) or specific OEMs and models?